Portable computers, such as notebook computers, have became popular for their compactness and mobility. Efforts to enhance their system performance have been continuing. However, they have a fatal shortcoming of limited available usage time due to the limited charge capacity of their internal batteries. Thus, a need arises in that the available usage time of a portable computer on one battery charge must be prolonged as much as possible.
However, it is recognized that increasing the charge capacity of the battery used in the portable computer has limitations due to its restricted size. Thus, reducing the electrical power consumption of components of the computer is desirable as is reducing the power consumed by in the computer itself.
It has been proposed, for example, that the operating voltage of semiconductor devices be reduced from 5 Volts to about 3 Volts or less, and that an Advanced Power Management (APM) system be set up in order to reduce the power consumption of a portable computer system during its use.
As is well known, the APM system has the following features:
First, it lowers the system clock frequency when the computer is not in use for a predetermined time after the computer system has been accessed. Secondly, it automatically shuts off the power supplied to displays, such as an LCD (liquid crystal display), when no keyboard or mouse input, or no changes of an image on the display have occurred for a preset time period. Lastly, it suspends the motor drive operation of a hard disk drive if the hard disk drive has not been accessed for a preset time period.
This power saving scheme is based on shutting off the power directed supplied to the energy consuming components of the computer system, if the computer is not in an interactive state for a predetermined time period.
Meanwhile, in the battery powered computer system, such as a notebook computer using an LCD display panel, it has been found that most of the power consumption occurs in the hard disk drive. In order to reduce the power consumption of the hard disk drive, the computer system which adopts the above APM system controls the driving of a spindle motor or servo motor provided in the hard disk drive by using commands that control the hard disk drive.
When the power saving operation for the hard disk drive is performed in the notebook computer, frequent ON/OFF operations of the driving motor occur due to the cache operation provided between the hard disk drive and the main memory of the computer system.
A disk cache is invisibly added to the system. A disk cache performs read buffering and write buffering between the hard disk drive and the main memory.
In caching write operations, many disk cache adopt write through algorithms to maintain so-called cache coherency and to prevent the loss data held in the cache due to the accidental or intentional power loss. The write through caching operates such that every time a change of data occurs in the cache, it manages to operate the hard disk drive to write the changes in the hard disk.
Therefore, even if the hard disk drive entered into the power saving mode by the APM system that allows the drive motor to stop, when the data in the cache has changed, the motor must be driven again to perform the write operation for the changed data.
At the start of driving a spindle motor of the hard disk drive, a transient period occurs in which the current supplied to the motor varies, until it reaches its normal rotating speed. During the transient period, the supplying current I.sub.t is larger than the normal supplying current I.sub.s. Thus, when the motor is driven to start rotating, it requires relatively large amounts of current I.sub.t.
Consequently, frequent power on operations from its off state of the drive motor will cause excessive battery consumption, and under some circumstance it will lead to the worst case where a power saving cannot be realized at all.
The following patents each disclose features in common with the present invention but do not teach or suggest the specifically recited method of controlling a hard disk cache for reducing power consumption of the hard disk drive used in battery powered computers in accordance with the present invention.
U.S. Pat. No. 5,475,859 to Kamabayashi et al., entitled Disk Cache Unit Haivn Reduced Waiting Times For Data Transfer, U.S. Pat. No. 5,600,816 to Oldfield et al, entitled, System And Method For Managing Data In A Cache System For A Disk Array, U.S. Pat. No. 4,835,678 to Kofuji, entitled Cache Memory Circuit For Processing A Read Request During Transfer Of A Data Block, U.S. Pat. No. 5,371,872 to Larsen et al., entitled Methods And Apparatus For Controlling Operation Of A Cache Memory During An Interrupt, U.S. Pat. No. 5,509,133 to Nishimukai et al., entitled Data Processing System With An Enhanced Cache Memory Control, U.S. Pat. No. 5,347,642 to Barratt, entitled Cache Memory Management Unit, U.S. Pat. No. 5,257,370 to Letwin, entitled Method And System For Optimizing Data Caching In A Disk-Based Computer System, and U.S. Pat. No. 5,535,358 to Kimura et al., entitled Cache Memory Control Circuit And Method For Controlling Rerading And Writing Requests.